The present invention relates generally to an electronic ballast for powering a discharge lamp. More particularly, the present invention relates to an electronic ballast having associated circuitry for controlled preheating of a discharge lamp.
An electronic ballast as known in the art for lighting a hot-cathode type discharge lamp, such as a high-pressure discharge lamp also referred to as an HID (High-intensity discharge lamp), typically includes a power converting circuit for receiving DC power and outputting AC power and a control circuit for controlling the power converting circuit.
In an example of an electronic ballast as previously known in the art, the control circuit performs an electrode heating operation by increasing the output frequency of the power converting circuit to heat each electrode of the discharge lamp. This heating operation takes place after a lamp starting operation and before lamp steady-state operation. The electronic ballast of this example can stabilize discharge after a shift to the steady-state operation and suppress lamp fade-out as compared to the case where the electrode heating is not performed.
Referring to FIG. 18(a), a starting period P1 is shown during which a starting operation is performed. When a subsequent electrode heating period P2 is relatively short as shown, an electrode of the discharge lamp is not sufficiently heated before the start of a steady-state period P3 during which a steady-state operation is performed. This results in a lamp current becoming asymmetrical between polarities. When the electrode heating operation shifts to the steady-state operation where the electrode of the discharge lamp is not sufficiently heated, as described above, discharge becomes unstable, thereby possibly causing fade-out. Accordingly, it is necessary to set the electrode heating period P2 to be sufficiently long as shown in FIG. 18(b). The necessary length of the electrode heating period P2 (duration of the electrode heating operation) varies for each discharge lamp.
However, where multiple discharge lamps may be connected to the ballast, a particular duration of the electrode heating operation that accommodates all discharge lamps is likely to be excessive for some of the lamps. Because the electrode heating operation allows the power converting circuit to output larger power than in the steady-state operation, to suppress a negative effect on the life of the discharge lamp the duration of the electrode heating operation needs to be reduced as much as possible while properly heating the electrode.